Method and device for monitoring multiple mirror arrays in an illumination system of a microlithographic projection exposure apparatus

ABSTRACT

Microlithographic illumination system includes individually drivable elements to variably illuminate a pupil surface of the system. Each element deviates an incident light beam based on a control signal applied to the element. The system also includes an instrument to provide a measurement signal, and a model-based state estimator configured to compute, for each element, an estimated state vector based on the measurement signal. The estimated state vector represents: a deviation of a light beam caused by the element; and a time derivative of the deviation. The illumination system further includes a regulator configured to receive, for each element: a) the estimated state vector; and b) target values for: i) the deviation of the light beam caused by the deviating element; and ii) the time derivative of the deviation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, U.S. application Ser. No. 14/255,302, filed Apr. 17, 2014, whichis a continuation of U.S. application Ser. No. 13/556,408, filed Jul.24, 2012, which is a continuation of U.S. application Ser. No.12/506,364, filed Jul. 21, 2009, now U.S. Pat. No. 8,339,577, which is acontinuation of, and claims benefit under 35 USC 120 to, internationalapplication PCT/EP2008/000920, filed Feb. 6, 2008, which claims benefitof German Application No. 10 2007 005 875.8, filed Feb. 6, 2007; GermanApplication No. 10 2007 036 245.7, filed Aug. 2, 2007; U.S. Ser. No.60/954,150, filed Aug. 6, 2007 and 61/015,999, filed Dec. 21, 2007. Thecontents of international application PCT/EP2008/000920, U.S.application Ser. No. 12/506,364, U.S. application Ser. No. 13/556,408and U.S. application Ser. No. 14/255,302 are incorporated by referenceherein in their entirety.

FIELD

The disclosure relates to illumination systems of microlithographicprojection exposure apparatus, in which arrangements of beam deviatingelements, such as micromirror arrays, are used for variable illuminationof a pupil surface.

BACKGROUND

In illumination systems of microlithographic projection exposureapparatus, which are used for the production of finely structuredsemiconductor components, flat arrangements of beam deviating elementscan be used to manipulate the projection light to try to improve theimaging properties of the microlithographic projection exposureapparatus. One example of this involves so-called multi-mirror arrays,in which a multiplicity of micromirrors are arranged in an array, suchas in rows and columns. The micromirrors are movable, such as tiltableabout two axes provided perpendicularly to one another, so that theirsurface normal can be tilted into any directions starting from a neutralposition. This can allow for variable alterations in the illuminationsettings in illumination systems.

SUMMARY

In some embodiments, the disclosure provides a device and a method bywhich the angle setting of mirror elements of a multi-mirror array canbe ascertained effectively. For example, deviations of projection lightthat strikes a multiplicity of flatly arranged beam deviating elementscan be detected and measured, and these deviations can therefore bemonitored and regulated. Because variations of the surface of opticalelements, such as the shape or alignment of surface regions, for exampledue to thermal loads or the like, are generally of interest formonitoring the imaging properties and possibly correcting imagingerrors, many other applications may be envisaged for such methods anddevices.

In some respects, the basic concept of the disclosure is that inaddition to the projection light of the illumination system, to whichthe flat arrangements of beam deviating elements are exposed, at leastone measurement light beam from a measurement illumination instrument isdirected onto the beam deviating elements to be examined, so that thedeviation of the measurement light beam due to the beam deviatingelement can be recorded by a detector instrument. If it is assumed thatthe deviation of the measurement light beam by the beam deviatingelement and the deviation of the projection light incident thereoncorrelate with one another, then the deviation of the projection lightor the change thereof relative to a specified setting can be ascertainedby this separate measuring instrument. With the additional provision ofa separate measurement illumination instrument which generates thecorresponding measurement light beam, the extraction of useful lightfrom the projection light can be obviated, while checking anddetermination of deviation changes of the optical element to be examinedcan furthermore be carried out continuously during use of themicrolithographic exposure apparatus. This can involve merely thearrival direction of the measurement ray bundle or bundles beingdifferent from the arrival direction of the projection light beam orbeams, so that no mutual interference takes place.

An angle variation of the surface normal of an optical element's mirrorsurface to be examined, or the alignment of a corresponding mirrorsurface, may be monitored and examined by such a procedure.

A method and the device can be used for the examination of mirrorelements, such as the aforementioned multi-mirror arrays (MMAs).

The arrival direction of the measurement ray bundle may differ both inthe incidence angle with respect to the optical element's surface to beexamined, and in an azimuthal incidence direction. The term azimuthalincidence direction is intended here to mean rotation of the incidenceplane of the corresponding ray relative to a predetermined plane, forexample an incidence plane arranged in a north-south alignment.

If the incidence directions of the measurement light beam and theprojection light do not differ in the azimuthal incidence direction,then they at least differ in the incidence angle to avoid mutualinterference and make it possible for the measurement light beamreflected from the mirror surface to be recorded by a detector system.

If the incidence direction of the measurement light beam and theincidence direction of the projection light beam or beams do differ inthe azimuthal incidence direction, then there may also be a differencein the incidence angle of the optical element to be examined. This isnot however compulsory.

A difference of the arrival direction of the measurement light beam fromthe arrival direction of the projection light beam or beams in theazimuthal incidence direction is often desired, in which case rotationangles in the range of more than 30°, such as more than 60°, and inparticular a mutual rotation angle of 90° around the surface normal ofthe optical element to be examined, are possible. In the case of a 90°arrangement between the incidence plane of the measurement light and theincidence plane of the projection light, a particularly largeinstallation space can be provided for arranging the measurementillumination instrument and a correspondingly arranged detectorinstrument.

In order to ensure defined illumination of the optical element to beexamined with measurement light, and likewise to permit definedrecording of the changes in the measurement light due to the interactionwith the surface of the optical element, an optical system mayrespectively be provided between the illumination source and the opticalelement to be examined on the one hand, and/or between the opticalelement to be examined and the corresponding detector instrument on theother hand.

The measurement light may have any suitable wavelength, and lie eitherin the visible or in the invisible range. In general, light will beintended to mean any electromagnetic radiation.

The optical system of the measurement illumination source may includeone collimator or a multiplicity of collimators, such as in the form ofa perforated plate with an upstream microlens array, so thatcorresponding collimated measurement light beams are generated.

These collimated measurement light beams are reflected by the surface tobe examined and, by converging lenses correspondingly arranged in frontof the position sensors of the detector instrument, such as a lens arrayof converging microlenses, they may be imaged into the focal plane ofthe corresponding converging lenses as a far-field diffraction image orFourier transform. Corresponding position sensors may be provided therein the focal plane, for example 4-quadrant detectors or two-dimensionalposition-sensitive sensors, which establish a deviation of the lightcone striking the detector from a neutral position, which corresponds toa determined alignment of the surface of the optical element to beexamined.

In order to obtain more installation space, additional optics may beprovided between the optical element to be examined and the detectorinstrument, which make it possible to arrange the detector instrumentfar away from the optical element to be examined. Optics may furthermorebe provided which allow variable arrangement of the detector instrumentwith simultaneous sharp imaging of a surface region of the opticalelement to be examined. To this end, the corresponding imaging opticscan be configured so that the optical element's surface region to beexamined is imaged onto optical lenses assigned to the position sensorswhile satisfying the Scheimpflug condition.

At the same time, the corresponding optics desirably ensure that theincidence direction of the measurement light beam on the convergingdetector lenses of the detector instrument corresponds to the alignmentof the associated surface regions of the optical element, or the tiltangle of the mirror elements of a multi-mirror array. This may, forexample, be ensured by relay optics having two converging lenses.

The angular alignment of the mirror surface of an optical element may bedetermined continuously during use of the optical element, or theillumination system in which the optical element is arranged. Theascertained values may therefore be used for active control orregulating of manipulable beam deviation elements, for examplemicromirrors of a multi-mirror array.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the disclosure will become clear fromthe following detailed description of exemplary embodiments with the aidof the appended drawings, in which:

FIG. 1 shows a highly simplified perspective representation of amicrolithographic projection exposure apparatus;

FIG. 2 shows a side view of an optical element to be examined in theform of a multi-mirror array;

FIG. 3 shows a plan view of the optical element to be examined in FIG.1, with a representation of the measuring arrangement;

FIG. 4 shows a perspective representation of a measuring arrangement;

FIG. 5 shows a side view of a measuring instrument;

FIG. 6 shows a side view of a measuring instrument;

FIG. 7 shows a side view of a multi-mirror array encapsulated in ahousing;

FIG. 8 shows a perspective representation of an exemplary embodiment inwhich the tilts of the individual mirror elements of a multi-mirrorarray are recorded with the aid of a camera;

FIG. 9 shows a representation of a pattern which is suitable for use inthe exemplary embodiment represented in FIG. 8;

FIG. 10 shows a side view of an illumination system with a multi-mirrorarray;

FIG. 11 shows a summary of a calibrating instrument, which represents onthe one hand a calibration plate and on the other hand and the intensityprofile during the movement of a mirror element and the relationdetermined therefrom between the mirror element angle and the systemangle;

FIG. 12 shows a diagram of a control loop which may be used in order tomonitor and control beam deviation elements;

FIG. 13 shows a detailed diagram of the regulating algorithm shown inFIG. 12; and

FIG. 14 shows a structural diagram of a measuring instrument whichemploys a frequency multiplex method.

DETAILED DESCRIPTION

1. Structure of a Projection Exposure Apparatus

FIG. 1 shows a highly schematised perspective representation of aprojection exposure apparatus 10, which is suitable for the lithographicproduction of microstructured components. The projection exposureapparatus 10 contains an illumination system 12 which illuminates anarrow illumination field 16, which is rectangular in the exemplaryembodiment represented, on a mask 14 arranged in the so-called maskplane. The illumination system 12 contains a light source, by whichprojection light can be generated. Conventional light sources are forexample excimer lasers with the laser media KrF, ArF or F₂, by whichprojection light with the wavelengths 248 nm, 193 nm and 157 nm canrespectively be generated.

Structures 18 on the mask 14, which lie inside the illumination field16, are imaged with the aid of a projection objective 20 onto aphotosensitive layer 22. The photosensitive layer 22, which may forexample be a photoresist, is applied on a wafer 24 or another suitablesubstrate and lies in the image plane of the projection objective 20,which is also referred to as the wafer plane. Since the projectionobjective 20 generally has an imaging scale |β|<1, the structures 18lying inside the illumination field 16 are imaged on a reduced scale as16′.

The performance of such a projection exposure apparatus is determinednot only by the projection objective 20, but also by the illuminationsystem 12 which illuminates the mask 14. Besides the intensity of thelight beam striking the mask 14, its illumination angle distributionalso has an effect on the quality with which the structures 18 containedin the mask 14 are imaged onto the photosensitive layer 22. Depending onthe direction and size of the structures 18 to be imaged, differentillumination angle distributions have been found to be advantageous.Since various masks 14 are intended to be imaged by the projectionexposure apparatus 10, an illumination system with which differentillumination angle distributions can readily be adjusted would be ideal.To this end it is desirable for a pupil surface of the illuminationsystem 12, which crucially determines the illumination angledistribution, to be illuminated as variably as possible by a drivableoptical element.

2. Measurement Principle

FIG. 2 shows a schematic side view of an example of such an opticalelement, for the monitoring and control of which the device or themethod may be used. The optical element in FIG. 1 is a so-calledmulti-mirror array 26 that includes a multiplicity of small mirrorelements 28 which are arranged movably, such as tiltably, so that themirror surfaces 30 of the mirror elements 28 arranged, for example, nextto one another in rows and columns can be aligned differently. Incidentprojection light 32 can therefore be distributed by reflection from themirror surfaces 30 into a multiplicity of reflected projection lightbeams 34, the propagation directions of which can be selected freely bytilting the mirror surfaces 30 within predetermined limits. The termtilting in this context is intended to be understood as a rotationmovement about an axis which may essentially extend centrally through amirror element 28, at its edge or even outside the mirror element 28, sothat the alignment of the mirror surface 30 changes with respect to theincident projection light 32. The latter two alternatives are also oftenreferred to as “swivelling”. Depending on the embodiment of themechanical suspensions and actuators of the mirror elements 28,combinations of translation and rotation movements, which will bereferred to below likewise for the sake of simplicity as “tiltingmovements”, are also used in order to achieve a change in the alignmentof the mirror elements 28 and consequently also the propagationdirection of the reflected projection light beam 34. In many systems,the incident projection light 32 is furthermore subdivided intoindividual light beams by using microlens arrays before striking themirror surfaces 30, and is focused onto the mirror elements 28.

Such a multi-mirror array 26 may then be used in an illumination system12 of a microlithographic projection exposure apparatus 10 for variableillumination of the pupil surface, also abbreviated to pupilillumination. To this end the incident projection light 32 is deviatedby a sufficiently large number of mirror elements 28 so that a desiredlight distribution is generated in the pupil surface. The number ofmirrors has an essential effect both on the spatial fluctuations of thelight intensity and on the minimum diameter of the reflected projectionlight beams 34, from the superposition of which the pupil illuminationis formed. Optical design calculations have shown that at least 4000mirrors are desirable in order to obtain an intensity distribution inthe pupil plane, which is comparable in respect of its properties withthat of a conventional diffractive optical element. Since very smallvariations in the tilt angles of the mirror elements 28 have largeeffects on the pupil illumination and therefore on the illuminationangle distribution on the mask 14, the disclosure proposes to ascertainthe exact angle positions of the mirror surfaces 30 by measuringtechnology.

As may be seen in FIG. 3, in addition to the incident projection light32 i.e. the useful light used to illuminate the mask 14 from theillumination system 12 (also referred to as the objective ray bundle),an additional measurement illumination device is provided which directsmeasurement light 36, for example in the form of at least onemeasurement ray bundle, onto the mirror elements 28 of the multi-mirrorarray 26. Depending on the exemplary embodiment, to this end themeasurement illumination may generate one or more measurement lightbeams or measurement ray bundles which are directed onto the mirrorelements 28 either in a scanning fashion, i.e. successively, orsimultaneously for some or all of the mirror elements 28. Since theincidence directions of the measurement light beams are known,conclusions can be drawn about the alignment of the reflecting mirrorsurfaces 30 by measuring the emergence directions of the reflectedmeasurement light beams. This utilises the fact that the deviation ofthe projection light 32 is correlated with the deviation of themeasurement light 36. The reflected measurement light 38 consequentlycontains information about the tilt status and therefore about thealignment of the mirror elements 28. In the measuring arrangementrepresented in FIG. 3, the measurement light 36 is directed onto themirror elements 28 in a plane which is rotated by 90° about the surfacenormal of the reflecting mirror surfaces 30 relative to the incidenceplane of the incident projection light 32.

Continuous measurement of the alignment of the mirror elements 28 istherefore possible even during operation of the illumination system 12.This does not therefore entail down times of the projection exposureapparatus 10 for determining the alignment of the mirror elements 28.Since a fraction of the incident projection light 32 is not used fordetermining the alignment of the mirror elements 28, no light loss whichcould reduce the throughput of the projection exposure apparatus 10 isincurred.

FIG. 4 shows a prospective representation of details of the measurementprinciple. As FIG. 4 reveals, the incident projection light 32 strikesthe mirror surface 30 of a mirror element 28 at a particular incidenceangle α along an incidence direction 40. Together with the surfacenormal 42 of the mirror surface 30, the incidence direction 40 of theincident projection light 32 spans the incidence plane (xz plane) 44, inwhich the emergence direction 46 of the reflected projection light beam34 also lies according to the reflection law.

According to the representation of FIG. 3, in a yz plane 48 which isrotated azimuthally about the surface normal 42 by a rotation angle θ ofthe order of 90° relative to the incidence plane 40 of the projectionlight 32, incident measurement light 36 is directed along an incidencedirection 50 onto the mirror surface 28 and is radiated after reflectionby the mirror surface 28 as reflected measurement light 38 along anemergence direction 52 in the direction of a detector instrument. Inthis solution, the incidence direction 50 of the measurement light 36therefore differs from the incidence direction 40 of the projectionlight 32 at least in the azimuthal incidence direction, i.e. in theincidence plane. In addition or as an alternative, the measurement light36 may also strike the mirror surface 30 at a different incidence anglefrom the projection light 32.

This is represented by way of example for an incidence direction 50′ ofthe measurement light 36, which lies in the same incidence plane 44 asthat in which the projection light 32 strikes the mirror element 28, butmakes an incidence angle α′ with the surface normal 42 which differsfrom the incidence angle α of the incidence direction 40 of theprojection light 32. The reflected measurement light 38 is thereforealso radiated by the mirror element 28 along an emergence direction 52′at a different angle from the projection light 32. This arrangement withthe incidence direction 50′ of the incident measurement light 36 and theemergence direction 52′ of the reflected measurement light 38 alsoconstitutes a solution.

3. Exemplary Embodiments of Measuring Instruments

FIG. 5 shows an exemplary embodiment of a measuring instrument in whicha light source 54 of the measurement illumination instrument directslight onto a perforated plate 56. A multiplicity of point light sources58 are generated by the perforated plate. Downstream convergingcollimator lenses 60, which are or may be combined in the manner of amicrolens array, respectively form a collimator and thereby generate acollimated measurement light beam 62 from the light produced by theassociated point light source 58. The measurement light beams 62generated by the various converging collimator lenses 60 can travelparallel to one another.

As an alternative to this, a single measurement light beam 26 may bescanned over the multi-mirror array 26 by two galvanometer scanners or acombination of one polygon scanner and one galvanometer scanner. Thelight source, for example a VCSEL (see below), may also be pulsed sothat the light source only illuminates when a mirror element 28 isfound. The regions between the mirrors do not therefore contribute tothe signal which is recorded in a time-resolved way by the detectorinstrument.

The collimated measurement light beams 62 strike the mirror surfaces 28to be examined in the multi-mirror array 26, where they are deviated indifferent directions as a function of the alignment of the mirrorsurfaces 28. The reflected measurement light beams 64 strike a microlensarray which contains a multiplicity of converging detector lenses 66, inwhose rear focal plane position sensors 68 of a detector instrument arearranged. Owing to this arrangement the angles, at which the reflectedmeasurement light beams 64 strike the converging detector lenses 66, arein a Fourier relation with the position of the focal points on theposition sensors 68, onto which the reflected measurement light beams 64are focused.

Since these angles of the reflected measurement light beams 64 dependaccording to the aforementioned reflection law on the alignment of therespectively associated mirror elements 28 of the multi-mirror array 26,by recording the position of the focal points on the position sensors 68it is therefore possible to determine the alignment of the mirrorelements 28. For example, 4-quadrant detectors or two-dimensionalposition-sensitive sensors may be used as position sensors 68. In thisway, for example, a tilt angle range of from ±2 to ±3° for the mirrorelements 28 can be ascertained relative to a predetermined surfacealignment.

If the mirror surfaces 30 of the mirror elements 28 have a curvature,then measurement light beams 62 may be directed onto different points ofthe same mirror element 28. This may be done simultaneously orsuccessively, in the manner of a scanning method, even with the samemeasurement light beam 62. The curvature can then be determined from thedifferent deviations of the measurement light beams 62 for the variouspoints of the mirror surface 30. Another possibility for determining thecurvature consists, for example, by determining the focal point diameterof a measurement light beam 62 coming from a curved mirror surface 30 onthe position sensor 68, in determining the beam's divergence andtherefore the curvature of the mirror surface 30, if it is assumed thatthe divergence of the incident measurement light 36 is known.

By integration and temporal comparison of a signal on the positionsensor 68, with the assumption of a light source with constantintensity, a possible change in the reflection coefficient of the mirrorsurfaces 30 may furthermore be measured and degradation of the mirrorlayer may be inferred.

In order to be able to arrange the detector unit with the positionsensors 68 and the upstream microlens array with the converging detectorlenses 66 at some distance from the multi-mirror array 26, according toan exemplary embodiment according to FIG. 6, additional relay optics 70are provided. The relay optics 70, which are represented purelyschematically by two converging lenses 72 and 74, image the multi-mirrorarray 26 onto the arrangement of converging detector lenses 66. Therelay optics 70 permit a larger distance from the optical element'ssurface(s) to be examined, in this case the mirror surfaces 30, withoutrestricting the angle range to be examined. The relay optics 70therefore decouple the detected angle range of the tilt from thedistance of the position sensors 68 from the multi-mirror array 26. Inthis way, the measuring instrument may be arranged outside the beam pathof the illumination system 12, where sufficient installation space isavailable.

In the exemplary embodiment shown in FIG. 6, the position sensors 68 andthe microlens array with the converging detector lenses 66 are arrangedin planes 76 and 78 which satisfy the Scheimpflug condition with respectto a plane 80, in which the multi-mirror array 26 is arranged. TheScheimpflug condition is satisfied when the principal plane of the relayoptics 70 intersects in one axis with the plane 76 of the convergingdetector lenses 66 and the plane 80, in which the mirror elements 28 ofthe multi-mirror array 26 extend. Compliance with this condition,despite the planes 76 and 80 arranged mutually inclined, makes itpossible for them to be imaged sharply onto one another. Such anarrangement can therefore image a large region of the surface of theoptical element to be examined, or a multiplicity of the mirror elements28, equally sharply onto the converging detector lenses 66 of themicrolens array, and it allows a corresponding angular arrangement ofthe detector instrument. With such an arrangement the plane 80 in whichthe mirror elements 28 extend, which is inclined to the optical axis,can be imaged sharply onto the converging detector lenses 66 of themicrolens array.

Similarly as in the exemplary embodiment of FIG. 5, the position of thefocal point formed on the position sensor 68 changes as a function ofthe incidence angle, at which the associated measurement light beam 64strikes the converging detector lens 66. Owing to the imaging by therelay optics 70, however, this incidence angle is in turn proportionalto the tilt angle of the associated mirror surface, the alignment ofwhich is intended to be measured. Here again, through deviations of thefocal point in the position sensor 68 from a neutral position, whichcorresponds to a predetermined alignment of the mirror surface 30, it istherefore possible to draw conclusions about the tilt angle of therelevant mirror surface 30.

The disclosure makes it possible to determine the alignment of themirror elements 28 of the multi-mirror array 26 during operation of theprojection exposure apparatus. In the event of deviations of themeasured alignment from a setpoint direction, the relevant mirrorelement 28 may be readjusted until the desired setpoint alignment isachieved. This is a prerequisite for active control or regulating of themirror elements 28, as will be explained in more detail below.

4. Encapsulation of the Multi-Mirror Array

FIG. 7 shows a highly simplified representation of a multi-mirror array26 which, in order to protect against external effects, for examplepressure or temperature variations, is encapsulated in a housing 82inside the illumination system 12. The housing 82 has a transparentwindow 84, through which the incident projection light 32 and themeasurement light 36 can strike the individual mirror elements 28 of themulti-mirror array 26. After the individual ray bundles have beendeviated according to the alignment of the mirror elements 28, they passthrough the transparent window 84 of the housing 82 in the oppositedirection.

In order to reduce undesirable reflections and concomitant intensitylosses, the transparent window 84 bears one or more antireflectioncoatings 86 which are conventionally adapted to the wavelength of thelight passing through and the angles occurring. In the exemplaryembodiment, the antireflection coating 86 is designed so that theincident projection light 32, which arrives at the angle α, can passthrough the transparent window 84 with the least possible intensityloss.

So that the incident measurement light 36 which usually has a differentwavelength, and under certain circumstances as explained above strikesthe multi-mirror array 26 at a different angle β, can also pass throughthe transparent window 84 without causing perturbing reflections, apolariser 88 is inserted into the beam path of the incident measurementlight 36. The polarisation direction of the incident measurement light36 is in this case selected so that the measurement light 36 isessentially p-polarised in relation to the incidence plane of themeasurement light 36 on the transparent window 84.

The measurement illumination instrument is furthermore arranged so thatthe incidence angle β of the incident measurement light 36 is at leastapproximately equal to the Brewster angle. This is because if lightstrikes the interface of two optical media at the Brewster angle, thenthe reflected light will contain only the s-polarised component of theincident light. The p-polarised component of the incident light is thenrefracted fully into the other optical medium. Since the incidentmeasurement light 36 is entirely p-polarised in the present case, and itdoes not therefore contain an s-polarised component which could bereflected, the intensity of the reflected light beam with incidence atthe Brewster angle is zero and no measurement light 36 will therefore bereflected at the transparent window 84. With approximate incidence atthe Brewster angle, i.e. in the range of 5° around the Brewster angle,the reflected intensity is somewhat less than 5% of the incidentintensity owing to partial reflection of the p-polarised component. Theincident measurement light 36 can therefore pass through the transparentwindow 84 virtually without losses, even though the antireflectioncoatings 86 are optimised only for the incident projection light 32 butnot for the incident measurement light 36. It is however advantageousfor the wavelength of the incident measurement light 36 to be greaterthan the thickness of the antireflection coatings 86, which have beenoptimised for the incident projection light 32, since in this case theantireflection coatings 86 have no effect on the incident measurementlight 36.

Although the angles of the emerging ray bundles no longer correspond tothe Brewster angle after reflection by the mirror elements 28 of themulti-mirror array 26, they are however close to the Brewster angleowing to the small tilt angles of ±2-3° of the mirror elements 28, sothat a reduction of the undesirable reflections is also to be observedin the emerging ray bundles.

The encapsulated multi-mirror array 26 described in connection with FIG.7 may furthermore be configured with gas tight encapsulation, so thatthe mirror elements 28 of the multi-mirror array 26 are enclosed by aninert gas which is contained in the housing 82. As an alternative, thehousing 82 may be provided with gas connections (not shown in FIG. 7) inorder to exchange the inert gas. The gas exchange may take placecontinuously, i.e. event during illumination of the multi-mirror array26 with measurement light 36 and/or projection light 32. As analternative, the gas exchange of the inert gases may also take placewhen the multi-mirror array 26 is not being illuminated with measurementlight 36 and/or projection light 32.

All gases or gas mixtures which prevent a reaction on the mirrorsurfaces 30 of the mirror elements 28 of the multi-mirror array 26, orwhich delay it so that the mirror elements 28 are not compromised or atleast not essentially compromised in their reflection properties duringtheir intended service life, i.e. their reflection behaviour (forexample their reflection coefficient) does not change by more than 10%during this period, are suitable as an inert gas. For the possiblereactions of the mirror surfaces 30 or the coatings used there, thelight wavelength and the light intensity with which the multi-mirrorarray 26 is operated should in particular also be taken into account.The inert gas used may also depend on the light wavelength and the lightintensity.

Since the mirror surfaces 30 usually have coatings to increase thereflection, and degradation of such a coating may take place in airdepending on the coating material or the coating materials, for exampleby reaction with the oxygen in air, degradation is prevented byencapsulation of the multi-mirror array 26 by the housing 82 and asuitable inert gas contained therein. Furthermore, many more materialsmay be employed for the coating of the mirror surfaces 30 since anyreaction of the coating materials with air is prevented owing to theencapsulation of the mirror elements 28 and the use of inert gases. Forexample aluminium, amorphous or crystalline silicon, chromium, iridium,molybdenum, palladium, ruthenium, tantalum, tungsten, rhodium, rhenium,germanium may be used as coating materials or as a material for themirror elements 28, and coatings of mixtures of these materials mayfurthermore be produced. For example helium, argon or other noble gases,nitrogen or mixtures of these gases may be used as inert gases.Furthermore, the gases employed may also be used for temperature controlof the multi-mirror array 26, for example in order to cool it duringexposure to measurement light 36 and/or projection light 32. Thetransparent window 84 of the housing 82 which encloses the multi-mirrorarray 26 may include amorphous or crystalline quartz or for examplecalcium fluoride, or consist of these materials, depending on thewavelength used and depending on the intensity used for the measurementlight 36 and/or the projection light 32.

As an alternative to the use of an inert gas as described above in thehousing 82 which encloses the multi-mirror array 26, this may also beevacuated or the gas or gas mixture may be modified in respect of itspressure or its composition. By evacuating the housing 82 or bymodifying the gas pressure or the gas composition, an interferingreaction on the mirror surfaces 30 of the mirror elements 28 of themulti-mirror array 26 may likewise be prevented or delayed so that themirror elements 28 are not significantly compromised in their reflectionproperties during their intended service life.

5. Determination of the Alignment by Pattern Recognition

FIG. 8 shows another possibility for determining the alignment of themirror elements of the multi-mirror array 26. In this case a pattern,for example a luminous pattern, is reflected by the multi-mirror array26 and imaged in a camera 91. The luminous pattern may for example begenerated by illuminating a semi-reflective screen 90 which carries thepattern, or by illumination through a transparent sheet (similarly to aphotographic slide).

FIG. 9 shows a pattern suitable for the purposes of the disclosure byway of example. The pattern has a chequerboard alternation betweenbright and dark, the frequency of which increases continuously along thetwo screen axes x_screen and y_screen so that no two regions of thescreen 90 have an identical pattern. If a detail of the camera imagewhich corresponds to a mirror element 28 is observed, then a differentregion of the pattern will be visible in this detail depending on thetilt of the mirror element 28. With the aid of an evaluation unit which,for example, carries out an autocorrelation between the detail of thecamera image and the known pattern of the screen 90, the exact tilt ofthe mirror element 28 can thus be recorded. Since the camera can bearranged so that it records a plurality of mirror elements 28 and eachmirror element 28 individually shows a region of the screen pattern, thetilts of a plurality of mirror elements 28 can be determinedsimultaneously by this instrument.

Instead of an ordered pattern as shown in FIG. 9, a random pattern mayalso be selected so long as it has an autocorrelation function which isas narrow as possible.

Another possibility consists in providing different colour profilesalong the two screen axes x_screen and y_screen, and thus achievingcolour coding of the different positions of the screen. In theory, acolour-sensitive camera 91 or another colour-sensitive sensor with onlyone pixel would then be sufficient in order to determine the tilt angleof the mirror elements 28 by the method explained above. Since thecolour vector or RGB vector is already provided directly in commerciallyavailable digital colour cameras, the evaluation would also be extremelysimple and not very computation-intensive.

6. Calibration—First Exemplary Embodiment

FIG. 10 shows a simplified representation of an arrangement which allowscalibration of the measuring instrument according to a first exemplaryembodiment. The calibration constitutes a comparison between the actualbeam deviations of the reflected projection light 34, which is intendedto illuminate the pupil plane of the illumination system 12 with thedesired intensity distribution, and the signals recorded by themeasuring instrument. The calibration presented here may, however, alsobe used when the signals which describe the alignment of the mirrorelements 28 are provided not by the measuring instrument describedabove, but by other sensors or measuring instruments. In this context,for example, electromechanical, piezoresistive, inductive, capacitive oroptical sensors arranged for example on the multi-mirror array 26, whichso to speak record the tilt angle from the “inside”, may be envisaged.

In a pupil shaping part 92 represented in a very simplified way in FIG.10, of an illumination system 12, the projection light 32 generated by aprojection light source, for example an excimer laser, strikes amulti-mirror array 26 and is directed after reflection thereon throughpupil optics 94 into the pupil plane 96 of the illumination system 12.Since the packing density of a multi-mirror array 26 suitable for such apurpose does not conventionally exceed 90%-95%, so that there are nosegments or no undesirably reflecting segments between the individualmirror elements 28, in this exemplary embodiment the incident projectionlight 32 is focused by microlens arrays in smaller projection lightbeams onto the mirror elements 28, as is known per se in the prior art,for example from WO 2005/026843 A2.

Via a separate beam path, a collimated measurement light beam 62 isfurthermore directed at a larger angle α′ onto the multi-mirror array26. In the present exemplary embodiment, the measurement illuminationinstrument includes an arrangement of a plurality of semiconductorlasers, which emit light from their flat semiconductor surface. Withsuch a so-called VCSEL array 98 (vertical cavity surface emitting laserarray), each individual mirror element 28 of the multi-mirror array 26can be deliberately illuminated with a collimated measurement light beam62. To illustrate the individual switchability, the beam path of twotemporarily extinguished measurement light beams 62′ is represented bydashes in FIG. 10. After reflection by the multi-mirror array 26, themeasurement light beams 62 strike a position sensor 68, which isarranged in the focal plane of the converging detector lens 66, asreflected measurement light beams 64 via a converging detector lens 66.Owing to the converging detector lens 66, an angle change of thereflected measurement light beams 64 causes a displacement of the focalpoints on the position sensor 68 onto which the reflected measurementlight beams 64 are focused.

In order to calibrate the measuring instrument, the arrangementfurthermore has a projection light detector 100 which is arranged at aprecise predetermined position in the pupil plane, but in immediateproximity near the usable pupil aperture, such as at a distance of lessthan one fifth of the diameter of the pupil aperture. If the measurementof the tilt angles of an individual mirror element 28 is now to becalibrated, then only the corresponding mirror element 28 is tilteduntil the projection light beam 34 reflected by it strikes theprojection light detector 100 in the pupil plane. If a measurement lightbeam 26 is simultaneously directed onto the mirror element 28 to becalibrated, then the focal point's position thereby established on theposition sensor 68 can be stored as a calibration value in an evaluationunit.

In order to record nonlinearities, such as may be caused for example bythe pupil optics 94 or by curved mirror surfaces 30, it is advantageousto arrange a plurality of projection light detectors 100, such as fourof them, around the pupil aperture. The projection light detectors 100may also be designed as 4-quadrant detectors.

Once the tilt angles of each mirror element 28 have been calibrated inthe way described above, the measuring instrument may be used to monitorthe tilt of the mirror elements 28 and therefore the illumination of thepupil plane during operation of the illumination system 12, in order toreadjust the mirror elements 28 if need appropriate. In general, suchreadjustment will be expedient since high-frequency perturbations in therange of from 100 Hz to 1000 Hz, such as may be caused for example byvibrations of the mirror elements 28 due to air currents or acousticwaves, would lead to intolerable errors in the illumination of the pupilplane.

Incorrect illuminations, which result from slow drift movements betweenthe mirror elements 28 of the multi-mirror array 26 and the microlensarray which focuses the projection light 32 onto the mirror elements 28,may furthermore be recorded by the described calibration method. Thesewill initially not be recorded by the measuring instrument, since it issubject to other drift movements. Since an individual mirror element 28may be aligned at the projection light detector 100 even duringoperation of the illumination system 12, without thereby substantiallyaffecting the illumination of the pupil plane, the calibration may berepeated gradually at particular time intervals for each mirror element28 during operation. The slow drift movements will thereby be recordedand corrected. Depending on how large the fraction of the projectionlight extracted from the normal beam path of the illumination system 12can be, the time intervals may be varied or individual mirror elements28, some mirror elements 28 or all of the mirror elements 28 may becalibrated simultaneously in this way.

7. Calibration—Second Exemplary Embodiment

FIG. 11 illustrates an overview representation of another method forcalibrating the measuring instrument described in detail above. Thecalibration method according to this exemplary embodiment may also beused independently of the measuring instrument. Use may therefore beenvisaged when the signals, which contain information about thealignment of the mirror elements 28, are provided not by the measuringinstrument described above but by other sensors or measuringinstruments.

For example, the calibration method of this exemplary embodiment mayadvantageously be used in order to directly calibrate the controlvariables for driving the mirror elements 28, if a so-calledfeed-forward operation of the multi-mirror array 26 is selected, inwhich separate sensor or measuring instruments are not necessarilyprovided for feedback. As will become clear from the explanations below,this is based on the fact that the proposed calibration method may berepeated rapidly with little outlay in order to recalibrate possibleslow-acting processes such as drift, electrical charges, etc., and mayeven be carried out for individual mirror elements during an exposureprocess of the exposure apparatus 10.

According to this exemplary embodiment, regions 102 with a reducedtransmission of 50% are generated at particular positions of the pupilsurface. To this end, for example, a transparent are calibration plate104 may be arranged in or in the vicinity of the pupil surface. Thereduced-transmission regions 102 respectively have the size of a lightspot generated in the pupil surface by a reflected projection light beam34. They form a kind of calibration scale, which is arranged eitherfixed in relation to the optical axis of the illumination system 12 orreplaceably at an accurately established position and angularlyprecisely aligned. By suitable methods, the reduced-transmission regions104 may also be arranged on or in elements which are already present,for instance in the pupil optics 94.

In order to calibrate a mirror element 28, an intensity sensor is fittedinto a field plane, for example the objective plane or the image planeof the projection objective 20. The intensity sensor records theintensity profile 106 while an individual mirror element 28 illuminatesdifferent positions of the pupil surface by the reflected projectionlight beam 34 assigned to it along predetermined paths, for examplealong a coordinate axis (see the graph at the top right in FIG. 11).Such an intensity profile 106 is represented by way of example in FIG.11 for a movement of a mirror element 28, in which the light spotmigrates centrally over the pupil surface along the X axis, i.e. beyondthe optical axis. If the reflected projection light beam 34 coming fromthe mirror element 28 strikes a reduced-transmission region 104, thenthis will be registered as a drop in intensity by the intensity sensor.

With the aid of suitable arrangements of the reduced-transmissionregions 102 and a corresponding evaluation unit, which records theminima of the intensity profile 106 and assigns them to particularpositions inside the pupil surface with knowledge of the arrangement ofthe regions 104, the measurement signals of the measuring instrumentwhich simultaneously measures the alignment of the mirror elements canthereby be calibrated. A relationship will thereby be found between thetilt angle of the mirror elements 28, as determined by the measuringinstrument, and the absolute angle position of the illumination angle ofthe projection light 32, as indicated in the graph at the bottom rightin FIG. 10.

In an advantageous refinement, an angle-resolving intensity sensor willbe used instead of a normal intensity sensor in a field plane. In thisway it is possible to establish not only whether light is actuallystriking a point in the field plane, but also the directions from whichlight is striking this point. Since different directions in the fieldplane are associated with the positions in the pupil surface, aplurality of mirror elements 28 may even be calibrated simultaneously bysuch an angle-resolving intensity sensor. The respectively illuminatedreduced-transmission regions 102 should then lie as far apart from oneanother such that the intensity sensor can still resolve the associateddirections in the field plane with sufficient accuracy.

In order to prevent local intensity drops from occurring in thereduced-transmission regions 102 on the pupil surface during projectionoperation of the projection exposure apparatus 10, twice as manyreflected projection light rays 34 as would otherwise be provided willrespectively be directed onto regions 102 to be illuminated. Since thetransmission of these regions 102 is 50% as indicated above, the doublednumber of projection light rays 34 therefore generates the desiredintensity. In this way, a homogeneous intensity distribution can begenerated in the pupil surface despite the statically used calibrationplate 104.

A transmission reduction by 50% takes place in the exemplary embodimentdescribed above, such reduced-transmission regions 102 then having beenilluminated with twice the number of mirrors, i.e. with twice the numberof reflected projection light beams 34, in projection operation. Thereduced-transmission regions 102 may also be reduced to 1/n, where n isan integer greater than or equal to 2. In this case, the respectivereduced-transmission region 102 will then be illuminated with nreflected projection light beams 34 in projection operation.

These embodiments are recommendable when the individual reflectedprojection light beams 34 have approximately the same intensity. If theintensities of the individual reflected projection light beams 34 differsubstantially from one another, however, then n may also be a numberother than an integer. In this case the reduced-transmission regions 102will be illuminated with a plurality of reflected projection light beams34 in projection operation, so that the desired intensity in the fieldplane is achieved for the angles assigned to the reduced-transmissionregions 102.

As an alternative to this, the calibration plate 104 may be removed fromthe beam path during normal projection operation.

8. Regulation—First Exemplary Embodiment

So far, devices and methods have been described which are suitable fordetermining the tilt angle of the individual mirror elements 28 of amulti-mirror array 26. Once information about the tilt angles has becomeavailable, then it is desirable to ensure by a regulating system that aparticular setpoint value for the tilt angle is complied with asaccurately as possible. The average value of all the neutral settings ofthe beam deviating elements can be adjusted with an accuracy of 1/6000.Relative settings with respect to this neutral setting shouldfurthermore be adjustable with an accuracy of at least 1/500.

The adjustment time t_(set), in which the mirror elements 28 aredesirably aligned, is established by the times within which the pupilillumination is intended to be modified for viable operation of themicrolithographic projection exposure apparatus 10. These timestypically lie in the range of 10 ms-50 ms. This has a direct effect onthe bandwidth of the regulating system, i.e. the frequency with whichthe tilt angles of the mirror elements 28 are intended to be measuredand adjusted.

For a multi-mirror array 26 in which it is possible to rule outindividual mirror elements 28 being excited in vibration by neighbouringmirror elements 28 or external effects, under certain circumstancesactive attenuation may be obviated if the mechanical properties of themulti-mirror array 26 are stable enough for so-called forward-feedcontrol.

Repeated calibration of the individual mirror elements 28 willnevertheless often be expedient, since the relationship between thenormal vectors nv of the mirror surfaces 30 and the applied controlsignals sv may change over time owing to various effects. Thisrelationship can be expressed by the equation nv=K(t)*sv. The quantityK(t) in the most general case is a tensor, since the control signals svmay also affect one another, for example by electrostatic charges. Ifthe time dependency of the tensor K itself depends on externalparameters p, for instance the temperature, then these effects may bemeasured by a separate measurement pickup (for example a thermometer).The tensor is then a function not only of time t , but also of theparameters p (i.e. K=K(t,p)). The tensor K(t,p) may then be employed fordetermining the control signals sv, without another calibration havingto be performed.

Yet since in general nondeterministic effects can never fully besuppressed, repeated calibration may nevertheless be desirable. With anadjustment time of 10 ms, a calibrating rate of 1 kHz (i.e. one tenth ofthe adjustment time) may be desirable for a calibration measurement of atilt angle of an individual mirror element 28 for viable feed-forwardcontrol.

For a multi-mirror array 26 in which vibrations due to internal orexternal interference can no longer be ruled out, a closed control loopis recommendable instead. Typical natural frequencies of 1-2 kHz for thetilt oscillations of the mirror elements 28 imply measurement andregulating rates of 1-2 kHz (e.g., 10-20 kHz) for each relevantcoordinate of an individual mirror element 28. In a multi-mirror array26 with at least 4000 mirror elements 28, this leads to a measurementrate of more than 4 MHz per coordinate, such as tilt angles ortranslations.

To this end it is possible to use a control loop which, depending on themeasuring device sensor signal received, acts directly on the controlvariables s for controlling the tilt angle of the mirror element 28 sothat the setpoint value of the tilt angle is complied with as accuratelyas possible. For such a purpose, a so-called PID regulator isconventionally used which receives the regulating difference e as aninput signal, i.e. the deviation between the setpoint value and theactual value of the mirror angle. Depending on the setting of theproportional (P), integral (I) and differential components (D) of thePID regulator, the control variable s is then set accordingly, which inturn affects the actual value of the mirror angle. Such a closed controlloop is operated with the so-called regulating frequency f.

In relation to the regulating of the mirror elements 28 of amulti-mirror array 26, however, the following problems arise. On the onehand, differentiation of the sensor signal is often difficult since thesensor values of the measuring instrument are strongly affected byinaccuracy. Differentiation by discrete filters in the regulatingelement, which is responsible for the differential component (D), maytherefore lead to such strong noise amplification that the resultantregulating signal is unusable. On the other hand, the regulatingdifference e can be calculated only with the sampling frequency at whichthe measurement values for the tilt angle of the mirror elements 28 areprovided. Owing to the large number of mirror elements 28, for example afew thousand or even several tens of thousands of mirror elements 28,the maximum sampling frequency for an individual mirror element 28 isgreatly limited. Moreover, the control loop can be likewise operatedonly with a regulating frequency f which corresponds to this lowsampling frequency, which may lead to sizeable deviations from thesetpoint value.

FIG. 12 shows the regulating scheme of a control loop which uses amodel-based state estimator, and which does not therefore present thedisadvantages mentioned above. The model-based state estimator estimatesthe current tilt angle of the mirror elements 28 on the basis of a modeland with the aid of the sensor signals affected by possible inaccuracies(for example due to the measurement methods). To this end themodel-based state estimator calculates the estimated state vector, i.e.for example the estimated tilt angle x and the time derivative x_pointof the tilt angle, by an internal model from the sensor signals(affected by inaccuracy). The state vector may also include a pluralityof tilt angles and/or other position parameters of a mirror element 28as well as their dynamic behaviour, for example their time derivative.

This estimated state vector is then compared with the setpoint status ofthe system, i.e. the actual setpoint value of the tilt angle and itstime derivative. Even though the time derivative of the tilt angle ishere again determined by differentiation from the setpoint value of themirror angle, this differentiation presents no problem since thesetpoint value of the tilt angle is not affected by inaccuracy. As wellas the regulating difference e, the time derivative de of the regulatingdifference is also obtained from this comparison, and these togetherform the regulating difference vector (e, de).

This regulating difference vector (e, de) is now sent to a regulatingalgorithm which calculates the control variable s and sends it to thecontroller of the mirror element 28. The regulating scheme of thisregulating algorithm is shown in detail in FIG. 13. As may be seen fromFIG. 13, the regulating algorithm has three proportional elements bywhich the effect of the various regulating components can be determined.A first proportional element KP corresponds to the proportionalcomponent (P) of a PID regulator, in which the regulating difference eis only multiplied by a constant. A second proportional element KImultiplies the output signal of an integrator, which integrates theregulating difference e, by a constant, and it therefore corresponds tothe integral component (I) of a PID regulator. A third proportionalelement KD corresponds to the differential component (D) of a PIDregulator, in which the time derivative de of the regulating differencee, which is sent to the regulating algorithm as explained above, ismultiplied by a constant. All three regulator components are added andoutput as a control variable s.

Owing to the model-based state estimator, such a control loop may evenbe used in digital form with measurement signals strongly affected byinaccuracy, as is the case with a conventional PID regulator.

As a starting point for producing the model-based state estimator, it isrecommendable to use state estimators known from the literature whichcan be particularly suitable for taking stochastic inaccuracies of themeasurement signals into account for the estimation, and to adapt thesestate estimators according to the desired properties of the specificapplication. Examples of this are the Kalman filter, the extended Kalmanfilter (EKF), the unscented Kalman filter (UKF) or the particle filter.

Since such model-based state estimators can even output the estimatedstate vector (x, x_point) with a rate which is higher than the samplingfrequency of the measurement signal, the regulating can be carried outwith a high regulating frequency f despite the large number of mirrorelements 28 and the concomitant low sampling frequencies of eachindividual mirror element 28. Sufficient accuracy of the tilt angles ofthe mirror elements 28 can thereby be achieved.

In the case of a Kalman filter, distinction is made between a kinematicmodel variant which is based on a Taylor expansion of the current tiltangle, and a dynamic model variant which more precisely replicates thebehaviour of the system specifically in the time periods in which nomeasurement values are provided.

All the regulating elements may furthermore be provided in multiples orin common for regulating a plurality of mirror elements 28. All theregulating variables, for example the control variable s, as a vectorwhose number of components is equal to the number of mirror elements 28.

Implementation of the control loop by software or on an FPGA chip isalso suitable for such an application, since in particular themodel-based state estimator can thereby be configured flexibly.

9. Regulation—Second Exemplary Embodiment

One important aspect of an illumination system 12, which has amulti-mirror array 26 for illuminating a pupil surface, is the speed andthe accuracy with which the individual mirror elements 28 can beadjusted. The key data for the measuring instrument, which records thetilt of the mirror elements 28, are in this case dictated by the opticaland desired mechanical properties for the design of the pupil shapingpart 92 of the illumination system 12 and of the illumination system 12overall.

In some embodiments, the multi-mirror array 26 has in total 64×64=4096mirror elements 28, which respectively have to be driven separately intwo axes and the tilt angles of which need to be measured individually.With the solutions known to date in the prior art, such a large numberof mirror elements 28 cannot be adjusted with the requisite accuracy andwithin the short times dictated by the desired properties for modernprojection exposure apparatus 10. This is because each mirror element 28should be able to adopt a tilt angle of at least ±2°, if possible ±3°,in both axes about a given neutral position, the control system needingto be able to govern this angle range with a systematic accuracy ofapproximately 11 microradians and a statistical inaccuracy ofapproximately 140 microradians.

It is therefore desirable to measure the mirror positions and applycorrections by a control loop. A time of approximately 1 ms is in thiscase available for measuring the entire set of 4096 mirror elements 28,i.e. it is desirably possible to determine the tilt angle of eachindividual mirror element 28 in approximately 250 ns with the requisiteaccuracy. The tilt angles of the mirror elements should furthermore bemeasured respectively with the aid of a measurement light beam 62 (seethe exemplary embodiment of FIG. 10), the propagation direction of thereflected measurement light beam 64 after reflection by the mirrorsurface 30 providing information about the tilt angle. The object istherefore to determine the tilt angle of the reflected measurement lightbeams 64 rapidly enough.

To this end, as explained above, angles are converted into positions byusing Fourier optics, for example the converging detector lens 66, andthese positions are recorded on a position sensor 68. Owing toinstallation space restrictions, however it is possible only withdifficulty to use 4096 parallel detector instruments, as shown for theexemplary embodiment of FIG. 5. Thus without sizeable outlay, as is thecase in the exemplary embodiment shown in FIG. 10, only one copy of theposition sensor 68 and the Fourier optics may respectively be used. Withconsiderable outlay it is possible to install approximately 4 detectorinstruments, but 4096 detector instruments can hardly be installed atpresent. The aim is therefore to satisfy the desired properties for themeasuring and control instruments with only one position sensor 68 andFourier optics.

To this end, FIG. 14 shows the schematic structure of a device thatallows a multiplex method, which allows parallel and independentmeasurement of a plurality of mirror elements 28 with only one positionsensor 68.

As mentioned in the exemplary embodiment of FIG. 10, arrangements oflaser diodes, so-called VCSEL arrays 98, may be used as light sourcesfor the measurement illumination. Such VCSEL arrays 98 with a square orhexagonal grid of 64×64 grid points are already commercially available.By a matrix drive in which the anodes and cathodes are respectivelyconnected to one another in rows or columns, simultaneous independentcontrol of up to 64 laser diodes is possible, for example within a row.The light from the laser diodes is then focused onto the multi-mirrorarray 26 by the converging collimator lenses 60 of a microlens array,which is mounted fixed on the VCSEL array 98.

Various commercial solutions may be envisaged for the position sensor68. Position sensors 68, which are also known as PSDs for short, arealready commercially available with a bandwidth of approximately 4 MHzand noise which just still allows the requisite measurement accuracy(for example the 2L4 type from SiTek). Yet since the measurement timeper mirror element 28 can only be about 250 ns, this position sensor 68is already at the limit of its performance with respect to the speed ofthe measurement. Attempts are therefore already being made to findalternatives for the position sensor 68, which achieve the requisitespatial resolution and allow even shorter measurement times. Thisdisclosure therefore relates to all detector types; the data of the 2L4PSDs from SiTek may be used as a starting point.

From the published data of PSDs, amplifier circuits and analogue-digitalconverters, the theoretically achievable statistical position error maybe deduced. Since this is somewhat smaller than often desired, it isthus theoretically possible to analyse the mirror elements 28 by timedivision multiplexing. The laser diodes of the VCSEL array 98 areswitched on one after the other, so that only the measurement light beam64 reflected by one mirror element 28 respectively strikes the positionsensor 68. Owing to the finite bandwidths of the laser diodes and theposition sensor 68, however, the length of time usable for themeasurement is reduced to less than 100 ns. This method will be referredto below as “sequential”, because the light sources are switched onstrictly one after the other and only one respectively illuminates at atime.

In order to counteract the restricting factors, for example the signalrise time of the position sensor 68, the DC offset and the drift, aswell as the 1/f noise of the amplifiers, the laser diodes are notswitched on and off one after the other, rather they are operatedsimultaneously in groups of for example four or eight laser diodes. Theintensities of the individual laser diodes of a group, however, aremodulated sinusoidally with different frequencies. Owing to thephase-sensitive detection of the electrode currents of the positionsensor 68, i.e. its output signals, the components of the variousreflected measurement light beams 64 at the different positions of theposition sensor 68 are separated according to their modulatedfrequencies. Simply speaking, the method is similar to a measurementwith a plurality of lock-in amplifiers operated in parallel.

This reduces the effect of the signal rise time of the position sensor68 as a rigid limit for the performance of the measuring instrument.What is more, measurements of the tilt angles of a plurality of mirrorelements 28 are therefore possible in the range of the bandwidth andwith reduced amplitude. The effect of all DC effects, i.e. offsets,drifts and 1/f noise, is furthermore filtered out. AC coupling of theamplifiers, for example, makes it possible to obviate the differentialamplifiers needed for the PSD bias of the position sensor 68, so thattheir noise is eliminated. The effect of the “dead time”, in which thesystem stabilises and no measurements are possible, is also reducedconsiderably.

As a side effect of the AC coupling, the quality of the digitisation ofthe sensor signals of the position sensor 68 is also improved since themost recent generation of analogue-digital converters, which are desiredirrespective of the measurement method being used, achieve their highestresolution only with AC coupling.

As a departure from the lock-in principle known per se, the system ismodulated not with one frequency but with a plurality of frequenciessimultaneously. In this feature, the system is somewhat similar to aFourier interferometer.

A particular choice of the frequencies and the data acquisition timesmakes it possible to use strictly periodic boundary conditions.Error-free Fourier analysis even of very short data streams is thereforepossible, and it is not necessary to use smoothing or a multiplicative“window” as in the case of lock-in amplifiers.

Although the readily apparent advantages of the described method arealso confronted with a few disadvantages, these can easily be overcome:

The maximum light intensity, which a position sensor 68 can process, islimited. The luminance of each individual light source should thereforebe reduced when a plurality of light sources are shining at the sametime, so that the ratio of signal amplitude to noise is reduced. Thisamplitude loss is however compensated for by the longer availablemeasurement time, so that the limitation of the maximum light intensityon its own does not entail any additional measurement error.

Based on the choice of frequencies (see below), a bandwidth of more than4 MHz is desired. The more light sources are modulated simultaneously,the greater is the desired bandwidth. Owing to the finite bandwidth ofthe position sensor 68, the signal amplitude at high frequencies becomeslower and the statistical error therefore becomes greater.

Correct choice of the measurement frequencies is crucial for successfulimplementation of the technique. In order to avoid the window problem inthe Fourier analysis, the frequencies are selected so that a wholenumber of periods of each frequency is respectively measured in themeasurement interval. The limits of the measurement interval aretherefore periodic boundary conditions for all frequencies. A window isnot therefore necessary and the measurement signals are exactlyorthogonal, which prevents channel crosstalk in the Fourier analysis.

The VCSEL array 98 and its electronics do not exhibit a linearrelationship between drive signal and luminosity. In addition to themeasurement frequencies, the light field therefore also contains theirharmonics. If such harmonics of one laser diode coincide with themeasurement frequency of another laser diode (i.e. of another mirrorelement 28), then the measurement result assigned to this other laserdiode will be vitiated. No measurement frequency should therefore be amultiple of another frequency measurement. In order to ensure this, thefrequencies are distributed as prime numbers within the bandwidth.

In the specific example, it is essential to achieve a measurement clockof 250 ns per mirror. In the case of four simultaneously active lightsources, four frequencies are needed in order to analyse four mirrorelements 28 simultaneously. Such a measurement therefore lasts 1 μs(with neglect of the transient time which may be of the order of 200ns). The periodic boundary conditions are thus applied for frequencieswhich are multiples of 1 MHz. The first four prime number multiples of 1MHz are thus frequencies 2 MHz, 3 MHz, 5 MHz and 7 MHz. In the case ofeight simultaneous measurements, the interval is 2 ps long and themeasurement frequencies are 1, 1.5, 2.5, 3.5, 5.5, 6.5, 8.5 and 9.5 MHz.The optimal choice of the number of frequencies depends on the bandwidthof the position sensor 68. Simulations have shown that the optimum forthe Silek 2L4 and the requisite key data lies between four and eightfrequencies; the exact value may also be determined experimentally.Since the density of the prime numbers decreases with an increasingvalue, the bandwidth also increases with an increasing number ofmeasurement frequencies so that the evaluable signal amplitude of theposition sensor 68 decreases, which in turn compromises the accuracy.

Owing to the limited total intensity, which can be detected at most onthe position sensor 68, the luminosity of the light sources are alsodesirably selected so that the saturation limit of the position sensor68 is not exceeded. To this end it is expedient to drive the lightsources so that the maximum total intensity is as low as possible, inorder that the average power of each individual light source can be setas high as possible. Since the frequencies are established by the primenumber distribution and each light source has the same amplitude, themaximum total luminosity can be minimised by adjusting the relativephases. A nonlinear numerical minimisation has shown that a significantreduction in the maximum intensity can be achieved merely by suitablechoice of the phases. Expressed in multiples of the individual maximumintensity, the maximum is for example 2.93 for 4 light sources, 4.33 for6 sources and 5.87 for 8 sources.

A significant advantage of the proposed method, compared with thesequential sampling of the mirror elements (without modulation) asdescribed in the introduction, is that the speed of the position sensor68 does not represent a fundamental limit for the measurement accuracy.Arbitrarily fast measurements are possible in principle, althoughmeasurement accuracy suffers from the decreasing signal amplitude.

However, it is therefore conceivable (and readily achievable bycorresponding configuration of the evaluation software in thecomputation unit) to switch over between different speeds and accuraciesupon command. For example, a sampling time of 0.2 ms for all mirrors maybe set for the active attenuation of the mirrors, with a correspondinglylarge measurement error, and it is possible to switch back again into 1ms with full accuracy for the actual adjustment process. With purelysequential measurement, a measurement frequency of 0.2 ms would nolonger be achievable with the SiTek 2L4. Under given conditions (i.e.when sampling times of 0.2 ms are intended to be achieved with the 2L4from SiTek in order to attenuate actively), this method is therefore notonly an advantageous solution, but possibly even the only solution.

The following components are desirable for implementing the measurementmethod:

-   -   A measurement illumination instrument with a multiplicity of        light sources, which is provided with a suitable number of a        driver amplifiers so that groups of 4, 6, 8, etc. light sources        can be operated simultaneously.    -   Signal generators which can generate frequency- and        phase-correlated sine signals, their number being equal to the        number of light sources driven simultaneously. Generators        according to the DDS (direct digital synthesis) principle are        highly suitable for this.    -   Four analogue-digital converters, which digitise the        preamplified signals of the position sensor 68. Since this        technique is based on synchronous detection, the clock source of        these converters is to be derived from the same reference as the        clock of the signal generators.    -   A computation unit, which can evaluate the sensor signals of the        analogue-digital converters. Owing to the nature of the task, it        is expedient to use a programmable logic unit in its place, for        example an FPGA (field-programmable gate array). The computation        unit (FPGA) accordingly has the following main tasks:        -   Collecting the measurement data of the four ND converters.        -   Numerical generation of sine and cosine signals with            frequencies equal to those of the light sources.        -   Multiplication of the sine and cosine signals by the A/D            converter data. This gives eight products per frequency            used.        -   Summing the products over the measurement interval. These            sums give non-normalised 0° and 90° components, from which            the amplitude of the respective input signal can be            determined by quadratic addition.

The 2D angle position of the respective mirror is determined from theamplitudes by simple addition, subtraction and division. A comparativelylong time is available for these operations, since they only need to becarried out twice per mirror.

By utilising the computation units which are available in modern FPGAs,this task can be performed with moderate outlay in a single FPGA.

The block diagram of the electronic instrument, including the mainfunctions of the FPGA, can be seen in FIG. 14.

Only the computation-intensive operations are indicated in thecomputation unit 108. “MAC” units are multiplier-adder units, which areprovided in an 8*n-fold configuration, n being the number of lightsources which are modulated simultaneously with different frequencies.Except for the computation unit 108, which is implemented as “firmware”of an FPGA and is not manifested directly in hardware, the arrangementis very similar to that of a conventional “sequential” measurementmethod and can therefore be constructed economically. In this diagram,the function of the DDS units has already been integrated substantiallyinto the FPGA (D/A converter, bottom right), although this may also beimplemented with conventional DDS components.

10. Concluding Remarks

The measures and devices mentioned above in connection with theillumination of a pupil plane may also readily be used advantageouslyfor active masks, in which arrangements of micromirrors are likewiseprovided as switchable elements.

The multi-mirror arrays may likewise be replaced by other reflective ortransmissive elements, which make it possible to deviate incident lightin different directions into different subregions of the element byapplying a control signal. Such alternative structures could for exampleinclude electro-optical or acousto-optical elements, in which therefractive index can be varied by exposing a suitable material toelectrical fields or ultrasound waves. This variation of the refractiveindex may then be used in order to achieve the desired deviation of thelight.

Although the disclosure has been described with the aid of certainexemplary embodiments, it is readily apparent to a person skilled in theart that alternatives or modifications are also possible in relation tothe some of the described features and/or different combinations of theproposed features, without departing from the protective scope of theappended claims.

1. A monitoring device for monitoring an arrangement of beam deviatingelements positioned in a first plane, the device comprising: an array ofdetector elements positioned to receive light from the arrangement ofbeam deviating elements; and an optical system positioned between thearrangement of beam deviating elements and the array of detectorelements and configured to image the arrangement of beam deviatingelements positioned in the first plane to a second plane, wherein theoptical system defines an optical axis that is inclined relative to thefirst and second planes. 2-30. (canceled)